In the more recently developed semiconductor technology, shallow trench isolations have been used in high density memory and other semiconductor devices since the isolation provides simplified back-end operations such as packaging. This is in contrast to a bird's beak type LOCOS isolation which provides an uneven top surface (or topography) on a memory device and leads to poor photolithographic results due to focusing difficulties. Shallow trench isolation can be etched in the silicon between neighboring devices. It allows a device to be built closer together.
When a shallow trench isolation is used in a semiconductor device, a more planar surface on the device can be obtained due to the absence of formation of bird's beak. Based on the advantages of a tighter line definition and a greater planarity that are achieved by the shallow trench isolation, the isolation is very suitable for applications in sub-half-micron semiconductor processes.
Isolation of individual semiconductor devices without using the bird's beak can be carried out by etching shallow vertical trenches in the silicon between neighboring devices. In shallow trench isolation, trenches of about 0.3 to 0.8 micron deep are anisotropically etched into a silicon substrate by a dry etching technique. The active regions in a substrate are protected from the etch during the trench etching step. After the trenches are formed, a chemical vapor deposition oxide is deposited on the wafer surface and then etched back so that the oxide only remains in the trenches having its top surface at the same level as the original silicon surface. The etch back process is frequently performed by using a sacrificial photoresist method. This processing technique has the advantages of not requiring any bird's beak and that no encroachment is involved. When two devices on a substrate are separated by a trench, the electrical field lines have to travel a longer distance and change directions twice, so that the field lines are considerably weakened. Shallow trenches of submicron dimensions are therefore adequate for isolation to prevent the punch-through and latch-up phenomena.
The drawbacks of a shallow trench isolation is its more complicated process and the sharp corners formed both at the top and at the bottom of the trench. This is shown in FIG. 1 where a shallow trench 10 is formed in silicon substrate 12 having a sharp top corner and a sharp bottom corner 16. In a shallow trench isolation, it is difficult to obtain round corners at the bottom of a trench, and even more difficult to have a round corner uniformly across a wafer. The sharp corners 16, shown in FIG. 1, could produce dislocations in the crystal lattice and also current leakage. The top corner 14 of the trench also need to be rounded in order to avoid leakage current. The sharp corners can cause field crowding and gate oxide thinning problems which result in a double hump behavior in a current/voltage relationship. The gate oxide thinning problem can cause a thinner layer of oxide being grown at the corner location. During a subsequent thermal cycle, junction leakage can occur. An over-etch 22 can occur at the interface where the oxide meets the silicon substrate surface. This is shown in FIG. 2. The exposed sharp corner 14 shown in FIG. 1 will further enhance the gate oxide thinning problem. It is therefore desirable to provide a method of preventing the occurrence of over-etch.
It is therefore an object of the present invention to provide a method of forming shallow trench isolation with rounded and protected corners without the drawbacks or shortcomings of the conventional shallow trench process.
It is another object of the present invention to provide a method of forming shallow trench isolation that produces trenches that have rounded and protected corners such that field crowding and gate oxide thinning problems do not occur.
It is a further object of the present invention to provide a method of forming shallow trench isolation with rounded and protected corners such that dislocation does not occur at the bottom corner of the trench.
It is another further object of the present invention to provide a method of forming shallow trench isolation that has rounded and protected corners by a chemical vapor deposition technique for depositing oxide into the trench.
It is still another object of the present invention to provide a method of forming shallow trench isolation that has rounded and protected corners by first depositing a layer of pad oxide and then a layer of silicon nitride on the silicon substrate as a masking layer.
It is yet another object of the present invention to provide a method of forming shallow trench isolation that has rounded and protected corners by first forming a bird's beak field oxide under a nitride mask and then etching away the main body of the field oxide.
It is still another further object of the present invention to provide a method of forming shallow trench isolation that has rounded and protected corners by rounding the top corners of the trench with the beak portion of a bird's beak field oxide layer.
It is yet another further object of the present invention to provide a method of forming shallow trench isolation that has rounded and protected corners by a technique of anisotropically etching silicon substrate to a depth of less than 5000 .ANG. through a nitride mask.